Scientists from Michigan State (MSU) and Stanford universities, in a fresh look at world population dynamics, have revealed evidence that increased numbers of households, even where populations are declining, are having a vast impact on the world's biodiversity and environment.

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ARLINGTON, Virginia - Scientists from Michigan State (MSU) and Stanford universities, in a fresh look at world population dynamics, have revealed evidence that increased numbers of households, even where populations are declining, are having a vast impact on the world's biodiversity and environment.

Reduction in household size has led to a rapid rise in household numbers around the world and has posed serious challenges to biodiversity conservation, write Jianguo (Jack) Liu of MSU and Stanford colleagues Gretchen C. Daily, Paul R. Ehrlich and Gary W. Luck in the Jan. 12 advance online publication of the journal Nature. Biodiversity is threatened severely not only by increased numbers of households, but also by less efficient per capita consumption of natural resources, the researchers say. They cite examples that larger numbers of households require more use of natural resources for construction, and that smaller numbers of people per household use on average more energy and goods per person.

The decline in household size, say the authors, is due to factors such as a rising number of divorces, and more "empty nesters" because kids are leaving home. They note that around the world, the proportion of multi-generational households is declining steadily.

"Personal freedom and social choice may come at a huge environmental cost," says Liu, lead author for the Nature article. Liu studied the loss of panda habitat in China under a Faculty Early Career Development (CAREER) grant from the U.S. National Science Foundation (NSF), an independent federal agency that supports research in virtually all fields of science and engineering.

"Liu's work has provided insight into the ways people interact with their environment," says Tom Baerwald, senior science advisor in NSF's Division of Behavioral and Cognitive Sciences. "It's an excellent example of interdisciplinary inquiry that bridges the natural and social sciences."

A systems ecologist, Liu received his NSF CAREER award in 1997. He already had acquired significant background on the impact of household dynamics on giant pandas in southwest China's mountainous Wolong Nature Reserve when he approached Stanford's Paul Ehrlich, renowned for his population studies, with an idea: Expand the probe into a worldwide look at the impact of households on global biodiversity and environment.

"The numbers of households increased much faster than the size of the population at Wolong," Liu said. "This has important implications because given the same population size, more households mean a need to consume more resources, and therefore, a greater impact on the environment. What was discovered from the panda reserve helped me to conclude that considering population size and growth alone is not enough, and made me want to find out whether other areas in the world have similar phenomena."

Liu's newly formed team, including Ehrlich, Daily, an ecologist, and Luck, a postdoctoral associate, proceeded to evaluate household dynamics in 141 countries. Among those are 76 biodiversity "hotspot" countries - including Australia, Brazil, China, Italy, Kenya and the United States - that have high density areas of animal and plant species, and threatened by human activity. In these biodiversity hotspots, the research team found that between 1985 and 2000 the number of households grew annually by 3.1 percent, whereas the population increased only 1.8 percent. Meanwhile, the number of people living in a single dwelling dropped from 4.7 to 4.0. The scientists estimated that had average household size remained the same in hotspot countries during the 15-year period, there would have been 155 million fewer households overall, and therefore, less pressure on biodiversity.

In the 65 non-hotspot countries, similar trends were found, although the magnitudes of change were less. Nevertheless, Liu contends, an increase in household numbers in non-hotspot countries directly influences important biodiversity on a national and local scale. Indirectly, he says, global environment is affected in such patterns as more energy consumption and release of more greenhouse gases.

Even in regions where population size decreased, such as in New Zealand, the number of households increased substantially because of a reduction in household size.

In China's Wolong, a reduced average household size was tied directly to an increase in household numbers and a rise in the amount of fuel wood consumed by the local populace for cooking and heating, which has contributed to deforestation and loss and fragmentation of habitat for giant pandas.

The "double toll" a reduction in average household size takes on the environment, say the scientists, includes more land use and more materials consumed for construction, and a lower efficiency of resource use per person. In hotspot countries, where this trend is most prevalent, the authors believe there may be severe limits on efforts to conserve species, thus "degrading the ecosystem services that biodiversity delivers to humanity."

In the past, the business community took most of the heat for many environmental problems, Liu explains. "While there is still a need to reduce pollution and ecological destruction caused by factories and companies, this study provides a wake-up call, and suggests that efforts at the individual and household levels are also needed to reduce impacts on the environment," he says. Additionally, changes in government policies such as tax incentives for sharing housing and resources could be helpful to influence personal and household decisions and actions.

Liu will further study the complex interactions of government policies, household behavior and biodiversity that will take him back to China as part of a 2002 NSF grant he received from a Biocomplexity in the Environment program special competition.

Additional support for the Liu team findings authored in the Nature paper came from the National Institute of Child Health and Human Development (part of the National Institutes of Health).

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